1. Field of the Invention
This invention relates to semiconductor wafer processing systems and, more particularly, to an electrostatic chuck for retaining a substrate within such systems.
2. Description of the Background Art
Electrostatic chucks are known to the art for electrostatically attracting and retaining a substrate, such as a semiconductor wafer, during processing such as in a physical vapor deposition (PVD) processing system, chemical vapor deposition (CVD) processing system and etching system.
An embodiment of such an electrostatic chuck is disclosed in U.S. Pat. No. 5,117,121, entitled METHOD OF AND APPARATUS FOR APPLYING VOLTAGE TO ELECTROSTATIC CHUCK, patented May 26, 1992, Toshiya Watanabe et al. inventors. This patent is incorporated herein by reference. An electrostatic chuck generally includes a sheet or block of dielectric material, e.g., alumina-titanium oxide compound, aluminum-nitride, polyimide and the like, having a pair of electrodes embedded in the dielectric material. When a voltage is applied between the electrodes, the substrate is electrostatically attracted to the electrostatic chuck according to the Johnsen-Rahbek effect or Coulombic effect.
To electrostatically retain a wafer in a vacuum chamber of a semiconductor wafer processing system, the ceramic body of the electrostatic chuck is mounted directly, such as by bolts, to a metal pedestal which typically is made of stainless steel. The ceramic body and the metal pedestal have different coefficients of expansion, and since the ceramic body and metal pedestal experience thermal excursions during semiconductor wafer processing, the ceramic body and metal pedestal expand differentially, and such differential thermal expansion coupled with the direct mounting of the ceramic body to the metal pedestal can result in ceramic breakage with the attendant loss of a relatively expensive ceramic electrostatic chuck, and more significantly and more expensively, can result in the ruination of a partially processed semiconductor wafer residing on the ceramic electrostatic chuck during breakage.
Further, in many semiconductor wafer processing systems that utilize plasma enhanced processing, the pedestal itself is biased with an RF signal such that a wafer supported by the pedestal is biased with respect to the plasma in the chamber. To effectively bias a wafer, the RF energy is coupled through the electrostatic chuck to the wafer that is retained thereupon. By coupling the RF energy from the pedestal through the electrostatic chuck, some of the RF energy is dissipated by the ceramic and the chucking electrodes. To enhance the amount of RF energy coupled to the wafer, it is desirable to apply the RF power to an RF electrode that is very near to the wafer rather than apply the power to the pedestal. As such, high current capacity electrodes that are embedded in the ceramic body are needed. This can be accomplished by embedding relatively large, thick electrodes, i.e., electrodes having high current carrying capacity, and relatively large feedthroughs into the ceramic body of the electrostatic chuck to carry the RF power. However, such an electrostatic chuck assembly can result in unwanted heating of the ceramic body by the RF currents. Such heating also may result in ceramic breakage. To moderate the thermal gradients within the chuck and avoid ceramic breakage, the ceramic body is made relatively thick. However, by making the ceramic thicker, more RF energy is dissipated in the ceramic and any advantages gained by applying RF energy to an RF electrode in the chuck rather than the pedestal are lost.
Accordingly, there is a need in the art for an electrostatic chuck having a relatively thin body of ceramic, an RF bias electrode positioned relatively near the wafer and a body of ceramic which is not mounted directly to a metal pedestal.